The invention relates to high bypass turbofan engines and, more particularly, relates to controlled fan stream flow bypass for high bypass turbofan engines.
Current practice for high bypass ratio turbofan engines is to utilize fixed area exhaust nozzles for both the fan duct stream and the turbine exhaust stream. As lower fan pressure ratios are utilized to achieve improved propulsive efficiency and reduced noise levels, the mis-match between fan operation at cruise conditions and operation at take-off conditions increases. This mis-match can be addressed using conventional approaches, but with a significant increase in weight, cost and complexity.
This mis-match can be explained with respect to the aero-thermodynamics involved within the high bypass ratio turbofan engine. FIG. 1 illustrates the change in fan duct stream nozzle flow capacity when going from operation at a cruise Mach Number of 0.85 to static conditions experienced during take-off operation for an engine with a fan pressure ratio of 1.6 at cruise conditions. FIG. 2 illustrates the change in fan operation for static conditions that results from this change in nozzle flow capacity. This change in flow capacity and resulting change in fan operation is the result of losing the benefit of ram pressure ratio due to aircraft flight Mach Number at static conditions. The change in fan operation shown for this example is typical of many operational engines and results in an acceptable loss in fan stall margin and fan flutter margin. Fan flutter margin is the margin of fan pressure ratio between fan operation and the limits where aeromechanical instability occurs. Fan stall margin is the margin of fan pressure ratio between fan operation and the limits where aerodynamic instability occurs.
As the fan pressure ratio at cruise is reduced, ram pressure ratio makes up a much larger fraction of the nozzle pressure ratio, and operation at static conditions results in a much larger change in both nozzle flow capacity and fan operation. FIG. 3 illustrates the change in fan duct stream nozzle flow capacity when going from operation at a cruise Mach Number of 0.85 to static conditions experienced during take-off operation for an engine with a fan pressure ratio of 1.3 at cruise conditions. FIG. 4 illustrates the change in fan operation for static conditions that results from this change in nozzle flow capacity. The change in fan operation shown in this example is that which might be expected for future very high bypass ratio engines, and results in an unacceptable loss in fan stall margin and fan flutter margin. FIG. 5 illustrates how an increase in fan duct stream nozzle area can be used to restore fan operation with acceptable fan stall margin and fan flutter margin. However, as mentioned above, achieving an increase in fan duct stream nozzle area at static conditions using conventional approaches results in significant increases in cost, weight, and complexity.
Therefore, there exists a need to achieve the benefit of varying the fan duct stream area while minimizing the impact of weight, cost and complexity.